1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine of the lean-burn control type wherein the air-fuel ratio is controlled to become a target air-fuel ratio on the lean side rather than a stoichiometric air-fuel ratio; in other words, a type wherein a lean mixture is used.
2. Description of the Related Art
Generally, a basic fuel injection time is determined on the basis of engine speed and inlet pipe pressure or intake air quantity. The basic fuel injection time thus determined is corrected in accordance with engine cooling water temperature, intake air temperature, and so on to determine execution fuel injection time. On the basis of this execution fuel injection time, fuel injection is performed. In addition, a lean-burn control system is known in which the air-fuel ratio is controlled on the lean side rather than on a stoichiometric air-fuel ratio. Since the peak of NOx is normally set somewhat on the lean side, deviating from the stoichiometric air-fuel ratio, the air-fuel ratio in the lean-burn control system is controlled beyond a level corresponding to the peak of NOx and to the lean side for the purpose of reducing NOx so as to improve fuel consumption.
Japanese Patent Application Laid-Open No. 62-199943 discloses a system in which lean-burn control is performed by determining a lean correction factor on the basis of inlet pipe pressure and engine speed and multiplying the basic fuel injection time by the lean correction factor.
A pressure sensor for detecting inlet pipe pressure is accurate in low and medium load ranges where a degree of opening of a throttle valve is small; however, in a high load range, the change of output of the sensor is small as compared to the change of opening of the throttle valve. That is, the resolving power of the sensor becomes degraded. Particularly, while a vehicle is running at high altitudes (high-altitude atmospheric pressure
is lower than low altitude atmospheric pressure PAo), the output of the pressure sensor in the high load range (where inlet pipe pressure PM is substantially equal to the atmospheric pressure PA) changes little and not in proportion to the change of opening of the throttle valve. That is, an air quantity being sucked into a combustion chamber of the engine cannot be detected accurately in the high load range by the pressure sensor. Therefore, an adequate lean correction factor cannot be obtained in the high load range, with the result that lean-burn control cannot be performed accurately. Such a problem arises also where the lean correction factor is determined using an airflow meter for detecting intake air quantity rather than inlet pipe pressure.